To simplify, the invention is described in the context of folding the blades of the main support and propulsion rotor of a helicopter, which raises a problem of positioning the blades in azimuth.
This is because of the field from which the invention stems, however the scope and the general aspects of the invention are not be limited to this particular field.
On the contrary, the invention applies to any other field in which it is useful to be able to set into coincidence members that are to be coupled together within a connection for transmitting rotation.
That said, there follows an explanation of how the blades of the main drive rotor of a helicopter are generally folded.
The document “Aérospatiale; Super Puma AS332; Instruction Manual; 21: Optional Equipment; Issue 1994” discloses that the purpose of folding the blades is to reduce the overall size of a helicopter, and also to reduce its wind surface.
In that document, folding is performed manually by a team of three human operators acting together. It takes about 5 minutes to 7 minutes.
Firstly, the main rotor must be placed with its blades arranged in a predetermined azimuth position, and then the rotor brake is applied.
Thereafter, the locking of the blades in their functional position is deactivated. The blades are then moved manually using handling poles (each blade is pivoted about a hinge axis between the blade and the hub of the rotor).
Finally, the blades in the folded position are secured to the helicopter fuselage. Document FR 2 861 689 also mentions the subject.
Reference is made below to various documents relating to automatically folding the blades of the main rotor of a helicopter.
Document FR 1 587 156 describes drive for folding the blades of a convertible aircraft. Two outlet shafts are driven by one or the other of two motors, while two outlets from a linear linkage operates a mechanism for locking the rotor blades.
Document GB 781 356 describes a hydraulic mechanism for folding the blades of a helicopter rotor. In response to a device for locking the blades in the flight position, the hydraulic mechanism causes a piston to retract, which operates blade folding.
Document GB 1 036 028 describes a mechanism for folding helicopter rotor blades in which an electric motor is mounted tangentially to a blade hinge.
Document GB 1 059 638 describes a mechanism for folding the blades of a helicopter rotor. A motor is mounted on a hinge that enables a blade to be swung between its flying position and its folded position.
Mention is also made of a few documents relating to the main transmission gearbox (MTG) that is to be found in a rotorcraft between its power unit and the main rotor.
Document FR 2 402 123 describes devices enabling power to be transmitted between a drive shaft and a driven shaft. Such a power transmission device includes gearing of the angle gear box type and is mounted between the driving and driven shafts.
Document FR 2 670 553 describes a power transmission mechanism designed to be placed between a drive shaft and two assemblies that are to be driven. The first assembly is driven optionally, while the second assembly is continuously driven, either by the drive shaft or by the first assembly.
On a helicopter, that device transmits power firstly to the rotor, which must be capable of being driven in rotation, and secondly to ancillary equipment such as a pump or an alternator. The ancillary equipment must also be capable of being driven when the helicopter engine is not running.
Consequently, the purpose of automatically folding blades can be better understood.
The operation is complex and requires care. In particular, for safety reasons, folding must be done with a success rate of the order of 100%.
It can also be understood that the components involved in folding are the subject of severe standards and requirements, particularly in terms of weight, size, the environment, reliability, and long life.
There are numerous additional practical difficulties when it is also desirable for blade indexing (predetermined positioning in azimuth) also to be automatic.
At present, the few practical solutions for automatic indexing often lead to problems of reliability, large size, and on-board weight.
In the abstract, one approach for obtaining highly integrated automatic indexing with a high degree of effectiveness would be to pass via the existing main transmission gearbox.
To do this, it would suffice in principle to couple a drive member at the outlet from an indexing actuator with a member that is to be driven at the inlet of the main transmission gearbox.
The drive member connected in that way to the gearbox could then turn the rotor until the blades reach the azimuth position required for folding.
Conventionally, such a coupling would be provided by a connection via members of the type mentioned above.
However, in practice, such a connection via members raises problems when the constraints to be satisfied are very severe (e.g. in terms of weight, size, the environment, reliability, and long life).
This applies in particular for setting the projecting and concave portions of a helicopter blade indexing mechanism into coincidence, which raises practical problems that have yet to be solved.
Furthermore, specifically because of these problems, it has frequently been preferred in practice to adjust azimuth manually or via an external actuator of the crank-handle type.
One of the problems lies in that, for certain positions of the main rotor, engagement approach is not possible because the projecting portions (male portions) and the concave portions (female portions) of the drive member and of the member to be driven substantially face one another respectively.
Under such circumstances, firstly approach is blocked by the male portions of the drive member and of the member to be driven coming into abutment against each other. This can damage the connection if any attempt is made to force engagement.
Secondly, if the male portions of the drive member and of the member to be driven overlap in part but with an angular offset that is sufficient for it to be possible to force engagement, then the contacting inlet faces of these portions can be damaged.
This can make it difficult or even impossible for the azimuth position of the blades to be indexed automatically on a later occasion.
Faced with this problem, one approach would be to chamfer the contacting inlet faces (i.e. endpieces) of the projecting and concave portions both of the drive member and of the member to be driven.
Nevertheless, it is found in practice that situations arise in which achieving engagement remains impossible because the plane portions (extending substantially perpendicularly to the axis of rotation) between the chamfers of the drive member and of the member to be driven come into contact on the outlet side (e.g. actuator) and on the inlet side (e.g. MTG).
Faced with this problem, another approach would be to release the rotor brake momentarily so as to be able to turn the gearing of the transmission gearbox a little and reach a relative angular position in which engagement is possible between the concave and projecting portions of the drive member and of the member to be driven.
Nevertheless, that would require either direct manual intervention or else reactivation (e.g. of the transmission gearbox, connection of a crank handle) which goes against the objective of indexing the azimuth position of the blades quickly and automatically.
However, and above all, when folding under windy conditions, for example, any release of the rotor could lead to considerable damage to the aircraft, its environment, or nearby personnel.
There can also be doubts concerning the success that is achievable by such a small amount of turning, particularly in the event of reactivation. Although it does indeed serve to get out of one blocked situation, there is no guarantee that the new relative position of the drive member and of the member to be driven will be any more suitable for coupling.
To avoid the male portions of the members for coupling remaining facing one another during engagement, another idea on similar lines might be to cause the drive member of the motor-driven actuator to turn slowly.
However this is not possible in many cases. For example, if the actuator is driven by an asynchronous electric motor, then it would start too suddenly for this purpose.
In this circumstance also, the contacting inlet faces of the male portions could be damaged, e.g. by being hammered under the effect of rotation combined with the axial engagement forces.
It is emphasized again at this stage that although the invention is described in association with a connection via members (e.g. using complementary grooves) with coupling taking place by axial sliding, the invention also extends to other forms of connection via members and other modes of approach.